Tensions are rising in Pakistan. Crowds of people are descending on the capital Islamabad for a big rally against the government. And their arrival is provoking an unusually strong reaction from the authorities. We're joined from Islamabad by our correspondent Philip Reeves. And Phil, bring us up-to-date on what's happening there.

PHILIP REEVES, BYLINE: Well, all day, two big convoys of trucks and buses and bikes and rickshaws and cars has been rattling across the landscape towards the capital Islamabad. They're bringing in thousands of people for what the organizers hope will be a massive protest against the government. It's taken them quite a while to get here. They set off yesterday from the city of Lahore 300 miles to the south. You can usually do that journey in a little over four hours by car. But it's taken them more than 24 hours. Why yesterday? It's because it was independence day in Pakistan - when the country celebrates its foundation and liberation from British colonial rule.

BLOCK: And who's leading the protest?

REEVES: Two men, both leading opponents of the government. One's the former cricket superstar Imran Khan. Khan's developed into a major political player in the last few years in Pakistan. He now heads one of the largest opposition parties. Pakistan's current government was elected last year by a landslide - the first time in Pakistan the power's actually been transferred from one democratically elected government to another. So that was a bit of a milestone. But Khan insists there was massive rigging, though there doesn't seem to be a lot of independent evidence to support that. So he's come here to call for the prime minister, Nawaz Sharif, to resign.

BLOCK: So one convoy being led by Imran Khan - and the other?

REEVES: Well, that's led by a charismatic religious grassroots leader. He's called Tahir ul-Qadri. He's very well known here because he heads a large network of schools and has a multitude of followers. Last year, in fact, he and about 50,000 of his followers held a four-day sit-in here in Islamabad. And this time, he's come with what he calls a revolutionary agenda, championing issues that affect the poor. And he wants things like free healthcare and free education - land reform. But he's also against Islamist militancy and has a plan to counter extremism.

BLOCK: Well, it sounds, Philip, like the government - the Prime Minister Nawaz Sharif is taking these protests very seriously - apparently sees them as really significant threat. What's at the root of that?

REEVES: Yes, strangely it's reacting as if, you know, this really is a big crisis. And in a lot of cities - capital cities - a demonstration of thousands or even tens of thousands wouldn't be a big deal. But here they're using big steel shipping containers to block roads. Many thousands of police have been deployed. Mobile phones have been cut in some areas. Shops and gas stations have been closed. There was some rock-throwing as the convoys were heading there. So that would be one reason why they're concerned about violence.

But there's another big theme here. You know, Pakistan's been on the map for 70 years. Much of that time's been under military rule. The military is still very powerful here. People worry that big protests and marches will lead to violence and chaos. And that could give the army a pretext to intervene again repeating history on the grounds that there's, you know, chaos and disorder. And they need to establish order. Although it's not at all clear that the army actually wants to do that.

BLOCK: And when you say intervene, the notion there would be there could possibly be a military coup as a result of the disorder.

REEVES: That fear is ever-present in Pakistan. But at the moment, people are more looking at it as if it's the military behind the scenes who are manipulating the opposition. Their goal, according to the speculation and there's no hard evidence for it, is to try to use the opposition to keep the civilian government of Nawaz Sharif in check.